Nowadays composite materials are commonly used for more or less specialized applications. These composite materials are generally manufactured, in a known manner, starting from a woven or nonwoven, organic or inorganic absorbent material, impregnated with a thermosetting resin. Application EP 0041054 notably describes the formation of such materials. Certain composite materials are constituted in particular of glass fibers, mineral fibers, cellulose or polyester fibers, impregnated with thermosetting resins based on urea-formaldehyde, phenol-formaldehyde, resorcinol-formaldehyde or melamine-formaldehyde combinations.
In general, preparation of these materials comprises impregnating the fibers of absorbent material with a solution of resin, a phenol-formaldehyde resin for example, in which microspheres consisting of a polymer material, of the vinylidene chloride/acrylonitrile type, containing an expanding agent of the isobutane type for example, are dispersed.
In particular, the example in the text of European patent application EP 01 02 335 notably describes a method for producing a composite material using cellulose fibers, said method comprising mixing microspheres with a suspension of cellulose fibers. After dewatering, the fibrous network is calendered and heated to 120° C. to initiate expansion. The expanded material is impregnated with an aqueous solution of phenolic resin, then dried by microwave and then the resin is crosslinked.
The method described in this application offers the advantage that the composite material can be produced in two steps, which can, to a certain extent, be spread over time. Conversely, it requires two baking phases, the first for causing the microspheres to react, which act as expanding agent and initiate expansion of the material, and the second for crosslinking the resin and imparting rigidity to the material. Therefore there is some complexity in application of this method.
It is therefore desirable to have an alternative, simpler method, by which a composite material, notably a composite material based on natural fibers, can be produced more easily.